1. Field of the Invention
The present invention relates to a powder-containing oil type lubricant applied for a die in the casting or forging processing of non-ferrous metals such as aluminum, magnesium and zinc, and to an electrostatic spray method and to an electrostatic spray apparatus using the powder-containing oil type lubricant.
2. Description of the Related Art
As is well known, the process using a die in the processing of non-ferrous metals involves methods including casting, forging, press working and extrusion casting. As viewed from the process, the casting is largely classified into high-pressure die casting, gravity die casting, low-pressure die casting, squeeze die casting and the forging is largely classified into cold forging and hot forging. Further, as viewed from the material to be the subject of the process, the material is largely classified into iron, non-ferrous metals and plastics. As viewed from the lubricant to be applied to the surface of a die, the lubricant is largely classified into a water based lubricant and an oil type lubricant, and the water-based lubricant is classified into a transparent solution type and a milky opaque emulsion type. As viewed from the components contained in the lubricant, the lubricant may be classified into a type containing a powder and a type containing no powder. As viewed from the spray method, it is largely classified into brush coating, liquid droplet coating and spray coating. The spray coating may be classified into combinations of a binary-fluid system and single-fluid system, and a non-electrostatic type and electrostatic type.
The high-pressure die casting, gravity die casting and low-pressure die casting are similar to each other in basic process. These processes are often likened to the process making an omelet by applying oil to a flying pan and by pouring a fresh egg into the flying pan. Specifically, when a non-ferrous metal is cast, a lubricant (corresponding to the cooking oil) is applied to the die (corresponding to the flying pan) to prevent the molten metal (corresponding to the stirred fresh egg) from sticking to the die. Then, dissolved molten metal at a high-temperature is poured in the die and solidified and then, the product (corresponding to an omelet) is taken out of the die. When viewing from the production efficiency and strength quality of the product in this case, the high-pressure die casting produces a low strength product with a high production efficiency, the gravity die casting produces a high strength product with a low production efficiency, and the low-pressure die casting produces a product having a strength closer to that of the gravity die casting than to that of the high-pressure die casting with a production efficiency closer to that of the gravity die casting than to that the high-pressure die casting. As products having the possibility of a danger to life caused by the breakage of parts, it is inevitable to produce those having high strength even with the low production efficiency. In the case of parts independent of life even if they are broken, the efficiency of production is regarded as important even if air is entrained to a form sponged part of which strength are reduced. Specifically, a main difference between these production methods is due to a difference in the rate of filling the molten metal in the die, and the filling rate is higher in the order of the gravity die casting, low-pressure die casting and high-pressure die casting. For this, the quantity of heat transferred to the coated film formed in the die is different depending on the casting method: the largest heat is transferred in the case of the gravity die casting and the smallest heat is transferred in the case of the high-pressure die casting. There is the case where the lubricant is decomposed or vanished corresponding to the transferred heat quantity and different lubricating technologies are applied to the die at present.
The forging process, on the other hand, is often likened to the process of producing a sword and is a method for raising the strength of the sword by beating a solidified metal. Namely, this process is also a method in which a solidified metal is beaten by high pressure to produce a desired shape. Though the time during which the coated film is exposed to a high-temperature environment is short, the coated film is exposed to very high-pressure. Accordingly, the lubricating technology is considerably different from that of the present casting process
It is difficult to satisfy all these requirements for the combinations of various classifications by one lubricant and therefore, individual technologies are applied to each use. However, lubrication technologies considering combinations of two or more of these classifications are possible. The present application relates to lubricating technologies aiming at the integration of these plural technologies and these lubrication technologies will be explained in the order of those for the high-pressure die casting, gravity die casting, low-pressure die casting and forging.
A) High-Pressure Die Casting
Seeing into this fields, 90% or more of the lubricants for non-ferrous metals such as aluminum, magnesium and zinc are water-based type releasing agents for the last forty years. Water-based releasing agents obtained by emulsifying effective components in water are applied to a die by the binary-fluid spray system mainly using pneumatic pressure. Electrostatic spray technologies have not been applied to water-based releasing agents due to excessively high electroconductivity at all.
Oil type releasing agents have come to be used which enables casting even if each of the releasing agent is used in an amount as small as 1/500 to 1/1000 that of the water-based releasing agent to be used from several years ago. However, the oil type lubricant can be applied only in a small amount and there is therefore the case where the coated film in a die, having a complicated structure or a large size and particularly at die positions hidden from the spray surface, may be insufficiently formed. In addition, because the die has surface irregularities, there is a tendency that a thick spray film is formed in the concave portion whereas a thin coated film is formed in the convex portion. For this, there is a tendency that oil type lubricant components are excessively retained causing an increase in casting porosity (sponging) in the concave portion, whereas the lubricity is insufficient, causing the seizure and soldering of the casting product with the die in the convex portion. As measures taken currently in the production site, the amount of the oil type lubricant is increased such that sprayed mists to be reached the hidden parts and the convex portion as much as possible to cast at a sacrifice of a small increase in casting porosity. Further, in the case of a large size die, the thermal energy of a molten metal of a non-ferrous metal is large. Therefore, the temperature of the whole die and particularly, the temperatures of narrow parts become close to the temperature of the molten metal and sometimes become 350° C. or more. For this, the oil type lubricant shows a “Lidenfrost” phenomenon and sprayed oil mists on the die surface boil. This causes the oil type lubricant to be deteriorated in the wettability of the surface of the die. Specifically, the boiling causes an increase in liquid droplets scattered on the floor from the surface of the die. As a result, t the coated film may become thin, bringing about deteriorated lubricity.
The measures taken in the case of the water-based release agent, it is applied in a large amount to cool the die surface, thereby sticking the release agent at a temperature less than the Leiden-frost temperature. This naturally causes a waste water problem. Two kinds of method are adopted as the measures taken in the case of an oil type lubricant. In one of these methods, a little more lubricant is applied to thicken the coated film. In the other, a small amount of water is applied so that almost all the water can be vaporized for cooling high-temperature narrow parts and then, the oil type lubricant is applied. If a little more oil type lubricant is applied, the thickness of the coated film at the parts, where a sufficient coated film can be formed, is increased as well. As a result, the amount of the casting porosity tends to increase. Because of this, the strength of the casing product is weakened a little. Besides, even though the amount of water is small, a pipe for coating is required.
Specifically, the prior art has the following problems.
(1) The oil type lubricant is insufficiently supplied to hidden parts of the die and it is therefore difficult to form a coated film necessary for lubrication at these parts.
(2) It is difficult to form a coated film having a satisfactory thickness at narrow parts of the die.
(3) It is difficult to form a uniform spray film at irregular parts of the die.
Electrostatic spraying is effective means to solve these problems concerning the oil type lubricants. In a spray apparatus, oil droplets of the oil type lubricant are negatively charged and sprayed to the positively charged die surface. The electrostatic spraying is the technology enabling the sprayed lubricant oil droplets to reach hidden parts of the die. However, electrostatic spraying cannot be applied in the case of a water-based release agent since it has excessively high electroconductivity. Japanese patent Application Laid-open (JP-A) No. 9-235496 relates to technologies used as the measures taken to impart conductivity to a paint to thereby drop the electric resistance by adding an alcohol or ammonium salt as an electrostatic assistant agent. However, alcohol or ammonium mists are not preferable at the casting site. JP-A No. 2000-153217 relates to technologies which hint the addition of an electrostatic assistant agent to a paint. However, “an electrostatic assistant agent having high polarity” is dissolved only in an amount of 0.3% by mass in “an oil type lubricant having low polarity”, and an electrostatic assistant agent tends to cause sedimentation and separation, which is not preferable. The present inventors have made studies concerning this problem, and as a result, found that at this level, the electrostatic assistant agent does not affect on the increase of adhesion amount of coated film. If a polar solvent is added, the dissolution of the electrostatic assistant agent may be increasingly solved. However, the health of a site worker may be damaged because of the polar solvent. For this, polar solvents are not preferable in the composition of the oil type lubricant in consideration of human health.
In order to solve the additional problems according to the electrostatic spraying as mentioned above, the present inventors have proposed such a technology that water and a solubilizing agent are blended in an oil type lubricant to impart slight conductivity for electrostatical spraying in the case of a high-pressure die casting. However, the technology tends to scarcely cope with the soldering caused by the deficiency of cooling ability originated from the very small amount of the lubricant to be applied.
(B) Gravity and Low-Pressure Die Castings
The flow speed of the molten metal during casting is an important factor for a coated film in casting. If the flow speed of the molten metal is extremely low similarly to the case of gravity die casting, the time during which the coated film is in contact with a molten metal having a temperature as high as about 600° C. is long, so that the coated film is significantly deteriorated. As a result, the coated film is thinned and there is therefore the case where the molten metal is stuck to the die surface when it is solidified. Therefore, the so-called “mold wash” prepared by suspending inorganic powders in water is mainly used at present in order not to be affected by the thermal deterioration. A coated film of the mold wash consists of inorganic powders and is not deteriorated. However, this needs drying because the mold wash contains water. Metaphorically expressing, this process corresponds to the plastering process used for Japanese houses and long-time drying is required. In the case of casting, molten aluminum and water give rise to steam explosion when the molten metal is poured into the die before the mold wash is completely dried up. For this, it is essential to carry out a drying process for several hours after the spraying is finished and this series of operation “spraying, drying and producing in each casting” extremely reduces production efficiency. In light of this, “the mold wash is sprayed once every tens or hundred and tens of products” to minimize the drying process at present. Further, the mold wash is considered as a craftsman technique and a skilled craftsman can produce 100 or more products per one time coating. An unskilled craftsman can sometimes produce only 10 products at most. Further, there is the case where a thick coated film made using the mold wash is partially peeled off. The peeled powder gets mixed in the casting product and extremely reduces the strength of the casting product. Because it is unclear when the peeling occurs, all casting products in the lot containing the peeled product is regarded as rejected goods and withdrawn from customers. Further, in view of product appearance, if the coated film is peeled off, the peeled part forms convexity, exhibiting inferior appearance.
In the casting process, it is important not only to prevent soldering but also to keep a perfect flow so that the molten metal can reach to finely engraved cavity parts of the die to make a product having a desired finish. In order to secure this molten metal flow, a thick coated film is formed. Specifically, it is so designed that the cooling of the molten metal is retarded and the viscosity of the molten metal is kept low for giving a good flow of the molten metal to fine parts of the die. Though the mold wash is applied once every tens of casting operations as mentioned above to secure a thick coated film (tens to hundred and tens of micrometers), a small amount of powder is intermingled in the casted product in each casting operation. For this, the coated film is gradually thinned, leading to reduced insulating efficiency. Finally, the temperature of the molten metal is dropped, and therefore, the flow of the molten metal cannot be secured, with the result that the flow of the molten metal into all parts of the die is inhibited. Namely, this metaphorically corresponds to the production of an omelet which loses its shape. The coated film is thick in its initial stage and is thin after tens of castings are finished. Therefore, the cooling rate of an initial product is different from that of a product obtained after tens of casting operations. As a result, the crystal structures of the metal are different from each other, bringing about the drawback that there is a difference in quality between a product obtained in the initial stage of the spraying and a product obtained in the later stage. Specifically for stabilizing the quality of a casting product, frequent spraying is required, but frequent drying is also required. This leads to reduced production efficiency. A thick coating film is formed in the first stage and is used until the lubricity is deteriorated to thereby decrease the number of inefficient drying steps at a sacrifice of stable product qualities.
Moreover, in the case of powder rich coated film, a casted product generally has a satin finished surface, which may fail to satisfy the requirement of quality of appearance depending on the product and it is therefore necessary to carry out after-treatment with the view of giving glossiness. In addition, the scattering of the powder after dried cannot be avoided because 100% (excluding the amount of water) of the powder is used, and it is necessary to take care of the working environment.
The technologies described in JP-A No. 2007-253204 and JP-A No. 2008-93722 are known as the technologies that compensate such a drawback. Both technologies relate to an oil type lubricant containing no water to remarkably reduce drying time. Further, the number of sprayings is increased to avoid excessively thick coated film, thereby forming a more uniform coated film than in the case of the usual mold wash. Moreover, the content of powders is reduced to make a film as thin as possible to prevent the peeling of the film. Further, this oil type lubricant contains a low-concentration powders and therefore, the scattering of the powders at production site is limited to minimum.
(C) Forging
The forging is a measures for compressing a metal material to be made into a product by deformation. This measures is largely classified into free forging and die forging. A sword made by beating an iron material without using any mold is a good example of the free forging. On the other hand, forging while making a uniform product using a mold is the die forging. The crank shaft of an engine part is a good example of the die forging. There is also the case where a material to be forged (hereinafter referred to as a work”) is heated to soften the material, thereby reducing the compressive force required for deformation. The heating temperature differs depending on the work material. Although the forging is usually classified into cold forging, warm forging and hot forging by the degree of heating, it is not clearly divided by numerical value.
The cold forging is carried out at a temperature (usually, ambient temperature) lower than the recrystallization temperature of the work material and has high dimensional accuracy. Therefore, many products can be developed without any after-treatment. The cold forging is suitable to small-sized products. On the other hand, the hot forging is carried out at a temperature higher than the recrystallization temperature and is applied to large-sized products. However, an oxide film is formed on the surface of the work and therefore, the crack of a product is easily caused. Further, the work is compressed under high pressure to deform. In the condition that no lubricant is present between the work and the die, scratching and soldering are caused between the work and the die under the high pressure. Therefore, a lubricant is applied to the die to prevent scratching and soldering.
Generally, a coated film is easily formed by physical adhesion in the cold forging. In the hot forging performed at high temperatures, on the other hand, the Leidenfrost phenomenon occurs at high temperatures and therefore, lubricant components are scarcely adhered to the die. Further, even if the lubricant components are adhered to the die, physical adhesion power between the both is low and it is difficult to form a good coated film. In the case of a lubrication using water as a medium, water can not be vaporized at 100° C. or less and no lubrication is therefore made; however, a coated film is easily formed at an intermediate temperature. However, if the temperature exceeds 240° C., a coated film is scarcely formed because of the Leidenfrost phenomenon.
As commercially available materials used to form the coated film, the following structures may be exemplified.
1) Graphite type: two types of lubricants, that is, a W/O emulsion type and oil dispersion type.
2) White powder type: W/O emulsion type of mica, boron nitride or melamine cyanurate.
3) Glass type: a mixture system of colloidal silicate and an alkali metal salt of an aromatic carboxylic acid (JP-A No. 60-1293) and a type which is used by diluting it in water.
4) Water-soluble polymer type: contains water (JP-A No. 1-299895)
Graphite exhibits excellent lubricity at temperatures ranging from a low temperature to a high temperature. However, in the case of graphite, the working circumstance is contaminated with a black powder and is inferior. Particularly, a lubricant of the type obtained by mixing graphite in oil is a cause of significant contamination. A lubricant mainly containing a white powder impairs the working circumstance not so much as graphite. However, if the content of the white powder is large, the working circumstance is also deteriorated. Further, the white powder is inferior in lubricity to graphite. In addition, there is the case where the white powder has a high hardness property, and there is therefore a tendency that the white powder damages the surface of the die to thereby shorten the life of the die.
Although a glass type or polymer type lubricant enables the formation of a thick film, it is inferior in lubricity to graphite and more reduces the life of the die than graphite. Further, the glass lubricant forms a glass film or polymer film around the equipment and periodical cleaning working is required though the frequency of the cleaning is not so much as in the case of a white powder, bringing about low working efficiency.
These graphite and white powder type lubricants are always concerned with the problem as to the occurrence of separation when they are stored and clogging of pipes and spray nozzles since these lubricants contain dispersed powders in water or oil. The water-glass type is dried in the vicinity of the nozzle. Particularly, when the working is suspended for a long time, the drying is promoted, causing clogging of the tip of the nozzle. As a result, when the working is restarted, the amount to be sprayed is reduced. Accordingly, the lubricating ability becomes insufficient, leading to the production of defectives. Though the W/O emulsion type lubricant is superior in the ability to cool the die, waste water treatment is necessary.
Further, when the surface of the die exceeds 230° C., mists of the lubricant embraced in water are boiled on the surface of the die. As a result, the adhesive efficiency of the lubricant to the die is impaired and it is therefore necessary to apply the lubricant in a large amount. Specifically, it is essential to severely control the temperature of the die because the formation of the coated film from the water-based lubricant is largely dependent on the temperature. Since water is scarcely vaporized at 100° C. or less, an emulsion type lubricant is unsuitable to cold forging. On the other hand, the emulsion type lubricant is used in warm or hot forging. However, water cools the die and the work to be forged heats the die. If this heating-cooling cycle is repeated, cracks are generated on the die surface. It is necessary to repair the die and in addition, an increase in the number of repairs results in the dumping of the die. That is, water shortens the life of the die. Further, when a drop in the temperature of the work in the molding process is significant, it is necessary to carry out molding under a heavy load, which is cause of shortened die life.
With regard to the method of spraying the lubricant, there is the problem that if the lubricant is applied in a large amount, the cycle time (the working time for producing one product) is prolonged. In the case of a water-based lubricant, the lubricant is applied in a large amount, which is not preferable in view of production efficiency. Further, there are problems concerning deteriorations in working circumstance caused by the scattering of the lubricant which is sprayed a large amount of the lubricant and concerning an increase of the frequency of the supplement of the lubricant for production. Moreover, there is the case where the heating process of work brings about a reduction in productivity. The production process using a conventional water-soluble lubricant is diversified after the temperature of the work is raised and the subsequent process involves, for example, pre-molding, course molding and finish molding. At this time, a resistance to deformation is increased, making it difficult to mold because the temperature of the work is dropped with the progress of the molding process. In the case of, particularly, a water-soluble lubricant, the amount of the lubricant to be applied becomes large, so that the die is cooled to accelerate a drop in temperature. To cope with this problem, there is the case of adding a reheating process. However, this reheating process causes an increase in cycle time, space and running cost, resulting in reduced production efficiency.
In order to solve the problems, the present inventors have proposed an oil type lubricant containing a low-concentration powder. It includes no water because the lubricant is oil type, and therefore the reduction in the deterioration of productivity and increase in production cost caused by water can be prevented. Further, because the concentration of the powder is low, the deterioration in site circumstance and the problem concerning sedimentation, when the lubricant is stored, can be reduced. Moreover, the cooling ability is small because the lubricant is applied in a small amount, so that the reheating process can be eliminated, exhibiting high production efficiency. However, scratching or soldering is caused under a heavy load though depending on the conditions.
In individual cases, prior art is established to some extent. However, the lubricating technologies which are common to the high-pressure casting, gravity casting, low-pressure casting and forging nowhere are disclosed.